1. Field of the Invention
The present invention relates generally to volume holograms and more specifically, to a volume hologram with increased Bragg angle sensitivity.
2. Description of the Prior Art
Holograms are recordings of light intensity patterns created by the interference of two beams of mutually coherent light (the two beams are usually obtained by splitting a single laser beam). There are two major categories of holograms: transmissive and reflective. These two categories are further divided into two physical types of holograms: surface relief holograms and volume holograms. Surface relief holograms can be recorded using photolithographic processes. The interference pattern is recorded as a periodic variation in thickness of the material while the refractive index of the material remains fixed.
In a volume hologram, the interference pattern is recorded as a periodic variation in the refractive index of the material while the thickness of the material remains fixed. The periodic variation in refractive index creates surfaces of peak refractive index within the material. These surfaces are referred to as "Bragg surfaces." When the interference pattern is created by two plane waves or two waves with identical curvature at the hologram surface, the Bragg surfaces will be Bragg planes.
When the hologram is re-illuminated by one of the original beams at an angle that results in maximum diffraction efficiency, the internal angle of the beam relative to the Bragg planes is referred to as the "Bragg angle." The external angle of incidence at which maximum diffraction efficiency occurs is also often referred to as the Bragg angle.
Recently, holograms have been used for optical data storage applications. These holograms allow beams of light to be separated for various purposes. These hologram systems include the following references: JP 1-13246, published Jan. 18, 1989; JP 1-55745, published Mar. 2, 1989; JP 1-55746, published Mar. 2, 1989; JP 1-86337, published Mar. 31, 1989; JP 1-86332, published Mar. 31, 1989; JP 1-146143, published Jun. 8, 1989; JP 1-53359, published Mar. 1, 1989; JP 63-25845, published Feb. 3, 1988; JP 62-219340, published Sep. 26, 1987; JP 61-123032, published Jun. 10, 1986; JP 1-18175, published Jan. 20, 1989; JP 61-42613, published Mar. 1, 1986; SU 1053056, published Nov. 7, 1983; EP 350,014, published Oct. 1, 1990; U.S. Pat. No. 5,013,107 , published May 7, 1991; and U.S. Pat. No. 4,497,534, published Feb. 5, 1985.
A problem with surface relief holograms is that they are not able to achieve efficient polarization separation except for a limited range of diffraction angles. Polarization separation is vex), important in magneto-optic systems where the data is sensed as changes in the plane of polarization of light reflected from the optical media. The ability of a hologram to have different diffraction efficiencies for beams incident at different angles is essentially controlled by the thickness of the diffracting structure. As a result, tile inherently thin surface relief holograms have a limited ability to separate beams propagating at small angles with respect to each other.
Volume holograms can be used to do polarization separation over a much wider range of diffraction angles. Volume holograms do not have the limitation problems of surface holograms since the whole volume, not just the surface, acts as the diffracting structure. However, volume holograms are limited ill thickness to approximately 25 microns or less. During processing, the processing chemicals have difficulty penetrating effectively beyond this depth. Thus, even if the holographic material is thicker than 25 microns, the effective hologram depth is limited to about 25 microns. The Bragg angle sensitivity of tile hologram increases as a function of the thickness of tile volume hologram. This means that a thick volume hologram is able to separate two beams having a very small angle of separation or divergence between their directions of propagation as compared to the ability of a thinner volume hologram. This ability to separate two beams having a very small angle of separation is especially important in multiple beam optical data storage systems. What is needed is a volume hologram of increased thickness and Bragg angle sensitivity.